The present invention relates generally to methods and apparatus for coating objects or moving webs by curtain coating and, more particularly, to an improved curtain coating method and apparatus for the manufacture of sheet goods including films and papers.
In curtain coating, a moving receiving surface is coated by the impingement of a free-falling curtain of liquid coating composition. The curtain may be a single coating composition or a composite of a plurality of layers of distinct coating compositions such as is described in U.S. Pat. No. 3,508,947 to Hughes. The coating receiving surface can be any surface that can be passed through a curtain. The receiving surface may be a continuous web of paper, plastic, metal, or cloth. In applications demanding high coating uniformity, such as the coating of photographic films and papers, the web must be stabilized by a precision backing roller to prevent vibrations of the tensioned web.
It is desirable to start and stop coating without the deposition of excess coating liquids on the coating receiving surface. Excess liquids necessitate additional dryer capacity and, therefore, result in additional costs. Coating composition that does not dry can result in contamination of the coating machine and cause laps in wound rolls of coated web to adhere to one another.
A method and apparatus for making starts and stops in curtain coating with minimal excess coating liquids on the receiving surface is described in U.S. Pat. No. 4,851,268 to Kozak. The startup of the curtain coating process is achieved by using a downwardly inclined catch pan device closely spaced to the backing roller and positioned to intercept the falling curtain before and during retraction of the pan. As shown in FIG. 1, the prior art catch pan 10 has two spaced lips on its trailing end, a primary lip 12 that retains the curtain liquid puddle formed on the device during coating startup and a secondary lip 14. There is a pan extension area 16 between the two lips 12, 14 that captures the curtain liquids extended from their free fall position as the primary lip 12 intercepts the curtain during its retraction. Preferably, the height of the two lips 12, 14 and the distance between them is determined by the retraction speed of the catch pan 10 and the time it takes the curtain to fall the distance from the top of the primary lip 12 to the pan extension area 16. The method requires passing the catch pan 10 through the curtain at high speed (50 to 200 centimeters per second) in the direction of the moving web. A disadvantage of this catch pan 10 is the risk of collision with the coating receiving surface because there is only a small gap therebetween. Another disadvantage is that it is possible for the catch pan 10 to outrun the curtain. That is, the curtain fails to contact the pan extension area 16 and secondary lip 14 because the vertical distance between the primary lip 12 and secondary lip 14 over which the curtain drops is too great.
In U.S. Pat. No. 5,017,408 to Kozak, a catch pan side wall geometry is disclosed that prevents excess liquids from being deposited on the edges of the coating receiving surface when starting or stopping. As shown in FIGS. 2 and 3, the sidewalls 18 of the catch pan 20 have flexible extensions 22 that make contact with the flushed edge guides 24 maintaining the curtain width. This contact removes the flushing liquid stabilizing the edges of the curtain 26. The curtain 26 may reform slowly or not at all on the dried portion of the edge guides 24 when the catch pan 20 is retracted. The edge guides 24 can also be contaminated or damaged by contact and their function impaired. The flexible edges 22 of the catch pan 20 may curl up when coating composition dries on them such that contact with the edge guides 24 is lost and excess coating solution passes onto the receiving surface 28 through the gap thereby created.
In U.S. Pat. No. 5,885,659 to Takahashi et al., a catch pan 30 as shown in FIG. 4 for use on a uniformly inclined upwardly running web 32 is disclosed wherein the pan 30 is retracted in the direction opposite to web motion. A receiving portion 34 of the pan 30 receives the curtain 36 before coating. A shelf portion 38 of the pan 30 receives the curtain 36 as the pan 30 is withdrawn to start coating. The shelf portion 38 and receiving portion 34 of the pan 30 are connected by an upright wall 40 of at least 8 millimeters that cuts the curtain 36 as the pan 30 is withdrawn and isolates the liquid in the receiving portion 34 of the pan 30. The shelf portion 38 is upwardly inclined from the receiving portion 34 by at least 5 degrees from horizontal to recover the liquid intercepted by the shelf portion 38. A disadvantage of this method is that the upright wall (level difference) 40 may have to exceed substantially 8 millimeters because of the tendency of the pooled liquids in the receiving portion 34 of the pan 30 to climb and spill over the upright wall 40 as the pan 30 retracts. The higher level difference necessitates a longer shelf 38. Also, retracting the catch pan 30 in the direction opposite to web motion causes any excess liquids that may reach the web 32 to pass through the curtain 36 and thereby disturb it. An additional disadvantage is that coating application must be performed on an unsupported web 32 under tension. This is generally not suitable for the most demanding applications.
The prior art catch pans are directed to coating receiving surfaces that are horizontal or slightly inclined. In European Patent Specification No. 0563308 B1 to Blake and Ruschak, a curtain coating method is disclosed in which the coating receiving surface is significantly downwardly inclined. Coating speed, as well as latitude in viscosity and coating thickness can frequently be increased by this method, and there is a need for a catch pan for a downwardly inclined coating receiving surface.
Typically, prior art catch pan systems include pneumatic cylinders positioned on each side of the catch pan. The pneumatic cylinders are used to drive the catch pan into and out of a curtain intercepting position. Pneumatic cylinders allow for a pan velocity on the order of 60 to 130 centimeters per second at the point the catch pan releases the curtain. The catch pan travels a distance on the order of about 20 cm through actuation of the pneumatic cylinders. The travel length and velocity require acceleration rates on the order of 2 times gravitational acceleration. Accelerating both sides of the catch pan at the same rate in order to maintain alignment using pneumatic cylinders is problematic at best. Prior art catch systems using pneumatic cylinders must be designed to allow for some undesirable skewing of the catch pan as it is accelerated into and out of the intercepting position.
The typical configuration of curtain coating apparatus often makes it impractical to tie the sides of the catch pan together rigidly enough to maintain alignment due to weight and the required acceleration rates. Further, mechanical linking mechanisms for tying the two pneumatic cylinders and therefore, the sides of the catch pan together are prone to misalignment, backlash, deflection, and wear. Attempts have been made to control the alignment of the catch pan while driving it with pneumatic cylinders by using equal lengths of supply and exhaust tubing for each cylinder as well as using precision pressure and flow control valves. However, even using these types of measures, the misalignment of the catch pan during movement is still on the order of several millimeters. Further, the speed achieved using pneumatic cylinders is inconsistent from cycle to cycle varying by 10% or more. The alignment and speed control of these prior art pneumatically driven systems is affected by friction from mechanical components and seals, deterioration of valves and controls, as well as dirt and wear.
It should also be understood that there is no controlled deceleration of the catch pan using pneumatic cylinders. Rather, the length of travel is limited in each direction by shock absorbing travel stops. The hard stops can ultimately result in damage to the catch pan and/or the travel mechanism. Further, the vibrations generated by the hard stops can have a deleterious effect on the coating operation.
It is therefore an object of the present invention to provide a catch pan for downwardly inclined coating receiving surfaces that is free of the limitations and disadvantages of prior art.
It is a further object of the present invention to provide a catch pan that does not deposit excess liquids on the receiving surface.
Yet another object of the present invention is to provide a catch pan that cannot be outrun by high retraction speeds.
Still another object of the present invention is to provide a catch pan that retracts in the direction of motion of the coating receiving surface and that permits the use of a backing roller at the point of coating application.
A further object of the present invention is to provide a catch pan that has a reduced risk of collision with the coating receiving surface.
Yet another object of the present invention is to provide a catch pan that intercepts the edge portions of the curtain without contacting the edge guides.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects, and advantages are accomplished by providing a catch pan that includes an intercepting pan segment that intercepts the curtain when coating is stopped, a substantially horizontal intercepting surface extending from the intercepting pan segment that intercepts the curtain when the catch pan is in motion, a release edge of the horizontal intercepting surface that passes close to the coating receiving surface at the point of curtain release, and two synchronized servo motors that drive the catch pan at high speed without lateral skewing. The horizontal intercepting surface overhangs the intercepting pan segment to form a capture chamber that has a volume of at least 1 cubic centimeter per centimeter of coating width in order to prevent excess coating solution in the intercepting pan from spilling onto the coating receiving surface. The bottom of the catch pan does not have to be closely spaced to the coating receiving surface. Rather, the small gap can be limited to the point of release of the curtain. Preferably, the speed of the catch pan as it is moved into and out of a curtain intercepting position, is at least about 100 cm/s and the releasing edge of the catch pan passes to within a distance of about 2 cm of said coating receiving surface at the point the curtain is released to impinge upon the surface of the substrate being coated.
Wire edge guides with suction removal means are also provided. The sidewalls of the intercepting pan segment have wing elements extending outward and upward toward the edge guides and terminating in a substantially horizontal severing edge gapped closely to the edge guides. Coating composition passing through the gap between the severing edge and the edge guide is drawn to the edge guide by surface tension and removed by the suction means so that the coating receiving surface remains dry until the catch pan is retracted. In addition to obviating the lateral skewing problem associated with driving mechanisms used to drive prior art pans, the two synchronized servo motors allow for controlled acceleration and deceleration of the catch pan in a non-violent manner.